A Pore-Centric Model for Combined Shrinkage and Gas Porosity in Alloy Solidification
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PREMIUM-QUALITY superalloy, titanium, ferrous, and aluminum casting suppliers for the aerospace and automotive sectors are competitively driven to produce higher quality and lower cost components, with shorter process development time. Both casting shrinkage porosity and gas porosity have long posed quality issues that can impair cast component performance associated with reduced fatigue life, reduced tensile and creep capability, and the potential occurrence of leakage-pathways for pressurized parts. Casting solidification software developers have been working with foundries to establish and implement porosity modeling modules that capture the fundamental physics of porosity formation, while respecting and adhering to the computational constraints imposed by the size and complexity of modern castings. Once verified and validated, these porosity models provide an opportunity to help guide casting development and to enable optimization of process parameters and gating/riser design, while also reducing the requisite number of empirically based casting development trials. The differences between the two types of porosity mentioned above (shrinkage porosity and gas porosity) are illustrated in Figure 1, which contains photographs of metallographic sections containing each type of porosity. Shrinkage porosity (Figure 1(a)) is caused by
VAHID KHALAJZADEH, KENT D. CARLSON, and CHRISTOPH BECKERMANN are with the Department of Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] DANIEL G. BACKMAN is with the Backman Materials Consulting, LLC, Saugus, MA 01906. Manuscript submitted June 20, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the density change from liquid to solid during solidification; it forms when the accompanying shrinkage can no longer be fed by flow of the liquid. It often forms late in solidification, when the solid dendritic network has a low permeability and is rigid. As a result, the porosity takes on the tortuous shape of the remaining spaces between the dendrites. Gas porosity (Figure 1(b)), on the other hand, occurs when the melt contains relatively large amounts of a dissolved gas. In this instance, pores can form much earlier in solidification, and therefore, the pores have the freedom to adopt a more spherical shape. The physics underlying the formation of both types of porosity are related, as explained in the next section, and there are certainly instances in metal casting where both mechanisms, shrinkage and gas, play a role simultaneously. Casting porosity has been the subject of numerous solidification research studies since the 1960s. Piwonka and Flemings[1] and Kubo and Pehlke[2] identified porosity formation mechanisms and mathematical models to describe porosity evolution. Over the next four decades, advances in the understanding and modeling of porosity formation were made by a number of solidification researchers.[3–10] These advances included the following: refining the description of the liquid pressure drop asso
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